1. Field of the Invention
The present invention relates to an optical system having a diffractive optical element used in a broad wavelength region such as a visible light region and, more particularly, to an optical system for projecting and displaying an image on an image display element and a projection optical apparatus using the same.
2. Related Background Art
In recent years, in order to achieve video display with presence or effective presentation, a large-scale, high-resolution screen display apparatus is demanded, and an optical system of a projection type image display apparatus is required to have still higher performance. The projection type image display apparatus includes a so-called three-plate type apparatus which uses three image display elements such as liquid crystal panels in correspondence with the red, blue, and green wavelength regions, and a so-called single-plate apparatus which displays a color image using a single image display element.
The single-plate apparatus has a simpler arrangement than the three-plate apparatus, and can attain size and weight reductions. The optical system of the single-plate apparatus includes an optical system in which color filters corresponding to light rays of the red, blue, and green wavelength regions on pixels of a single image display element such as a liquid crystal panel or the like are provided, and an optical system in which light rays having different wavelength regions such as red, blue, and green are caused to be incident on predetermined pixels on an image display element with different angles of incidence with one another.
When color filters are used, since each pixel transmits only the wavelength of a specific wavelength region, and absorbs other wavelengths, the incident light suffers a large loss, and it is difficult to realize a bright projection type display apparatus.
An outline of the arrangement in which light rays having different wavelength regions such as red, blue, and green are caused to be incident on respective pixels on an image display element with different angles of incidence with one another will be explained below.
FIGS. 1A and 1B show an outline of the arrangement of the aforementioned apparatus. A light source P101 emits white light. Light emitted by the light source is uniformed by an illumination light intensity distribution uniforming means P103, which is set on an optical axis P102, and emerges from the means P103 as a nearly collimated light beam. An optical unit P103 includes a so-called optical integrator.
A color separation means P104 comprises dichroic mirrors P104a, P104b, and P104c each having wavelength selectivity, and the light from the light source is deflected by the color separation means.
In the color separation means P104, the dichroic mirrors are arranged to make different angles with one another, so that an image display element P105 is illuminated with rays with wavelengths selected by the dichroic mirrors with different angles. The image display element P105 comprises, e.g., a transmission type liquid crystal element. Light rays transmitted through the image display element form images of the image display element P105 on a screen P107.
As described above, light rays ray_a, ray_b, and ray_c of the wavelength regions selected by the dichroic mirrors of the color separation optical system P104 illuminate the image display element P105 at given angles. The light rays ray_a, ray_b, and ray_c correspond to, e.g., those of the green, red, and blue visible light regions.
An array-like focusing means such as a microlens array P109 is formed on the image display element P105, and focuses the respective color light rays at different positions. An image display portion P110 of the image display element P105 has pixels P112, the amounts of light transmitted therethrough can be controlled by a control means (not shown) that controls the image display element P105, and pixels P112a, P112b, and P112c are arranged in correspondence with the focusing positions of the microlenses P109.
A projection optical system P106 projects those pixels P112 onto the screen.
With this arrangement, a projection optical system that can assure higher use efficiency of light than the system using the color filters can be realized. In such optical system, an optical system that projects an image is required to have higher performance to attain still higher resolution of the image on the screen. Since the solid angle of light from the liquid crystal panel increases due to use of the microlenses, a brighter projection lens is demanded.
A technique that improves optical performance using a diffractive optical element to meet such performance improvement requirements is disclosed in papers such as SPIE vol. 1354 International Lens Design Conference (1990), Japanese Patent Application Laid-Open Nos. 10-115777, 11-064726, and the like. These techniques exploit a physical phenomenon: chromatic aberrations with respect to light rays of a given reference wavelength appear in opposite ways on refraction and diffraction surfaces in the optical system. That is, this means that the diffractive optical element has negative dispersion (Abbe number xcexdd=xe2x88x923.453) while typical optical glass has positive dispersion. Also, the diffractive optical element has strong anomalous dispersion ("THgr"g, F=0.2956).
In addition, since an aspherical lens effect can be utilized by changing the periodic structure of gratings of the diffractive optical element, a great improvement of optical performance can be expected. Furthermore, since such characteristics of the diffractive optical element are obtainable by a microscopic shape, the space factor is very low, and weight and size reductions can be easily achieved. Exploiting such characteristics of the diffractive optical element disclosed by Japanese Patent Application Laid-Open Nos. 10-115777 and 11-064726, an improvement of performance and a size reduction of the optical system are attained.
As described above, when the diffractive optical element is used, effective features that cannot be realized by a refractive optical system alone can be provided. But since the diffraction efficiency of the diffractive optical element largely depends on the wavelength and angle of incidence, the diffraction efficiency of the element must be sufficiently taken into consideration.
FIG. 2 shows an example of the diffraction efficiency of a single-layered diffractive optical element formed of a given material. As can be seen from FIG. 2, the first-order diffraction efficiency at a specific wavelength (design wavelength) around 520 nm is high, but the diffraction efficiency of a wavelength separate from the design wavelength drops considerably. In the wavelength region that suffers such diffraction efficiency drop, diffraction efficiencies other than the design order such as the 0th order, second order, and the like increase, and cause image deterioration such as flare or the like.
In consideration of such diffraction efficiency characteristics of the diffractive optical element, an arrangement that reduces flare resulting from light rays of unnecessary orders is disclosed in Japanese Patent Application Laid-Open No. 08-220482.
FIG. 3 shows an outline of Japanese Patent Application Laid-Open No. 08-220482. The arrangement shown in FIG. 3 has an imaging lens system 3 including a relief diffractive optical element, and an illumination optical system 1. A diffractive optical element 11 has an effect of a single lens as a whole, while its relief pattern surface 10 is divided into a plurality of regions with different groove depths, which can maximize diffraction efficiencies with respect to a plurality of light rays of different wavelengths. The illumination optical system 1 comprises a wavelength selection element 9 (e.g., a band-pass filter) having a plurality of transmission regions, which respectively have, as center wavelengths of transmission, wavelengths that maximize the diffraction efficiencies of the divided regions, in correspondence with the divided regions of the relief pattern surface 10. The relief pattern surface 10 and wavelength selection element 9 are so disposed as to be nearly paraxially conjugate with respect to a lens system located therebetween.
Although such arrangement can reduce generation of unnecessary diffracted light rays, a light loss occurs since the wavelength selection element such as a band-pass filter or the like is used.
It is, therefore, an object of the present invention to provide an optical system having a diffractive optical element, which can solve the aforementioned problems, and can suppress generation of flare and the like due to light rays of unnecessary orders of diffraction caused by the wavelength dependence of the diffractive optical element.
The present invention provides an optical system having a diffractive optical element arranged as in (1) to (9) below:
(1) An optical system in which light from a light source is separated into light rays having predetermined wavelength regions by a color separation optical system, the light rays are caused to be incident on pixels in an image display element having a plurality of pixels, which correspond to the predetermined wavelength regions, and an image displayed on the image display element is projected in an enlarged scale by a projection optical system,
characterized in that a diffractive optical element is provided near a pupil of the projection optical system, and diffraction efficiency of the diffractive optical element can be optimized in each of predetermined wavelength regions.
(2) An optical system set forth in the above (1), characterized in that the light rays separated by the color separation optical system into the predetermined wavelength regions are caused to be incident on light focusing means provided in an optical path of the image display element at the light source side at different angles of incidence with one another, and are focused on each of the plurality of pixels of the image display element.
(3) An optical system set forth in the above (1) or (2), characterized in that the diffractive optical element is constructed to have a grating which has different heights in each of the predetermined wavelength regions in correspondence with the predetermined wavelength regions.
(4) An optical system set forth in the above (1) or (2), characterized in that the diffractive optical element is constructed to have a grating height of which changes continuously across the predetermined wavelength regions in correspondence with the predetermined wavelength regions.
(5) An optical system set forth in the above (1) or (2), characterized in that the diffractive optical element is formed of materials having different refractive indices in each of the predetermined wavelength regions in correspondence with the predetermined wavelength regions.
(6) An optical system set forth in the above (1) or (2), characterized in that the diffractive optical element is formed by combining the arrangement set forth in the above (3) concerning the different heights of the grating of the diffractive optical element, and the arrangement set forth in the above (4) concerning the height changing, in correspondence with the predetermined wavelength regions.
(7) An optical system set forth in the above (1) or (2), characterized in that the diffractive optical element is formed by combining the arrangement set forth in the above (3) concerning the different grating height or (4) concerning change in the grating height, and the arrangement set forth in the above (5) concerning employing the materials having different refractive indices, in correspondence with the predetermined wavelength regions.
(8) An optical system set forth in any one of the above (1) to (7), characterized in that the diffractive optical element has region where diffraction efficiencies can be optimized for at least two wavelength regions.
(9) An optical system set forth in the above (8), characterized in that wavelengths of the wavelength regions are those in visible light region.
(10) An optical system set forth in the above (9), characterized in that if xcex1 and xcex2 represent the optimized maximum and minimum ones of each wavelength region, a difference between xcex1 and xcex2 is not less than 50 nm.